The invention relates to a system for measuring the penetration depth of an elongated object such as a pile, tube, sheet pile or drill into the ground, to be used in combination with an installation for bringing the elongated object into the ground for instance by pressing, hammering, vibrating, drilling or lowering, or for pulling or otherwise removing said elongated object out of the ground, said system comprising means for measuring the displacement of the elongated object.
A system of this type, more specifically destined for driving a pile into the ground by means of a hammer, is described in the laid-open Japanese patent publication JP 58-94525. In this prior art system the displacement of the elongated object is measured by measuring the displacement of a wire or cable of which one end is directly or indirectly connected to the pile. The other end of the wire or cable is wound onto a drum. During operation of the system the amount of wire paid out from said drum is measured. The measured values are recorded and can be used for real time calculations. Thereby especially information is obtained about the speed with which the pile or tube is penetrating into the ground.
A specific problem with measurements of this type resides in the fact that installations for driving piles, tubes, sheet piles or drills into the ground are usually operating in a very dirty, even hostile environment. Therefore, the various components of the measuring system have to be embodied such that they will operate in a reliable manner even under said hostile circumstances. Rotating means such as a drum on which a measuring wire is wound (as described in JP 58-94525) should be avoided. A specific disadvantage related to the use of a drum resides in the fact that the error occurring during the successive revolutions of the drum is cumulating in the final measurement value, resulting in many cases in a relatively large absolute divergence between the measured value and the real penetration depth. Furthermore slip and stretching of the wire could lead to an additional deviation.
The object of the invention is now to indicate in which way accurate information can be obtained about the penetration of the pile or tube etc. with means which are basically insensitive for a dirty or even hostile environment.